Chapter 4 covered this topic in detail, but it's worth reviewing. Generically, for maximum cross-platform compatibility, stick with the Metal look and feel, and perform sanity checking to ensure that the user interface operates correctly on both platforms. For the best performance (and user experience) on Mac OS X, however, be sure to let users run the application with the Aqua look and feel. Doing so involves using appropriate fonts and spacing so that Metal and Aqua interfaces look good on every platform.
While Apple's Aqua GUI is excellent and the implementation allows first-class application appearances, the same cannot be said for the standard Windows look and feel. Determining whether you want to support one or more native look and feel targets is largely a matter of budget and resources (mostly consumed by the testing personnel). Whatever you decide, though, you need to test your GUI applications on every platform they will run on. This might mean buying some extra hardware (or better yet, salvaging those old 486 and Pentium II machines), installing Windows and Linux, and actually seeing what your application looks like on each platform. Despite the best advice from this book, things can go wrong when running an application on a platform it wasn't designed for or developed on. Your own eyes are always the best verification.